I'm lucky enough to have access to a long arm quilter now, but I spent years working with my vintage machine both using safety pins and spray basting. I used a lot more pins than you have, at least all four corners of each of the Sue blocks and probably 4 more in the middle of each side, maybe one in the bonnet. And then probably at least 2 in each long piece of sashing. My fingers were always hurting from all the pinning but my stuff did not move!
I'd carefully roll up the long sides and start out by doing the long center sashing. Then I'd turn it and do the middle sashing row. Then go back and do the other inner long sashings, turn again and do the short way, and then around the edge.
Once the sashing stitching is in and the quilt is what I called stablized, I'd take out the pins except for the one in the bonnet.
I really like Sam hiding out with all the Sues
